For example, in circulation cooling towers in large-scale air-conditioning facilities, condensation by evaporation of circulating water induces corrosion of metal parts in contact with cooling water, algae, and scale deposition. The condensed water therefore needs to be partially discarded, and the water to be discarded is separately treated and discharged. One of the proposed techniques for treating the condensed boiler cooling tower blowdown water is treating boiler cooling tower blowdown water with filters and reverse osmosis membranes (RO membranes) and supplying the treated water to the cooling tower again (Patent Literature 1).